1. Field of the Invention
The present invention relates to polyvinyl alcohol (hereinafter referred to as PVA) fibers having excellent hot water resistance and high strength and elastic modulus, and to a process for producing the same. The fibers of the present invention are suited for industrial uses including, particularly. reinforcement of composite materials.
2. Description of the Prior Art
PVA fiber has higher strength, elastic modulus, resistances to weather and chemicals, and adhesiveness than polyamide, polyester and polyacrylonitrile fibers and has developed unique uses mostly in industrial field. In recent years the fiber has caught much attention as reinforcement fiber for cement (substitute for asbestos fiber) because of its high alkali resistance.
If a PVA fiber having high resistance to hot water and high resistance to dry heat, as well as still higher strength and elastic modulus, is developed, rubber and plastics reinforced with such fiber and rope, fishing net, tent and the like comprising such fiber would become usable under severer conditions of high temperature or high wet temperature, thereby being superior materials excellent in safety, durability, light weight and like features.
The PVA polymer used for commercially available PVA fibers has a stereochemical structure of an atacitc body having a diad syndiotacticitY of 53 to 54% as determined according to the evaluation method of tacticity of the present invention which will be later described herein. Commercially available PVA fibers obtained from this PVA are insufficient in water resistance and wet heat resistance and cannot be said to have sufficiently high strength and modulus.
For the purpose of obtaining a PVA fiber having improved water resistance, fibers obtained from PVA having high syndiotacticity, or high water resistance in other word, have been persued. Highly syndiotactic PVA's have been obtained from polyvinyl trifluoroacetate and polyvinYl formate, and the obtained PVA's can be dissolved in solvents and be wet spun. See for example Japanese Patent Registration Nos. 539683, 548856, 581737 and 615659. However, although the PVA fibers obtained by these processes have higher water resistance and wet heat resistance than fibers utilizing conventional atactic PVA, the resistances are still insufficient for practical purposes and further the fibers have low strength of 9 to 11 g/d.
The use of highly syndiotactic PVA for the purpose of improving, besides water resistance, strength and elastic modulus of the fibers obtained therefrom has been proposed. Thus, Japanese Patent Application Laid-open No. 108713/1986 discloses a pProcess which comprises obtaining a PVA from vinyl trifluoroacetate, which has a diad syndiotacticity as defined in this specification of 58%, dissolving the PVA in dimethyl sulfoxide (hereinafter referred to as DMSO) or glycerine and conducting dry-jet-wet spinning to obtain fiber having a single filament strength of 15 g/d at most and elastic modulus of about 380 g/d at most. However, these values are of not so high level and the wet heat resistance is, like in the above Japanese Patents, not sufficient.
Japanese Patent Application Laid-open Nos. 130314/1984, 289112/1986 and 85013/1987 disclose processes which comprise using high-polymerization degree PVA to obtain fibers having a strength of 19 to 29 g/d, an elastic modulus of 550 to 650 g/d. These fibers are, however, cannot be said to have sufficient hot water resistance.
Crosslinking treatment is known for the purpose of improving the hot water resistance of PVA fiber. See for example Japanese Patent Application Laid-open Nos. 120107/1988, 156517/1989 and 207435/1989. However the crosslinking causes the resulting fiber to decrease its drawability, whereby the obtained finished fiber becomes insufficient in strength and modulus.